devops(backend): switch to multi-stage Docker build

Replace runtime mvn spring-boot:run with a proper multi-stage build:
- Stage 1 (builder): compiles JAR with BuildKit cache mount for ~/.m2
- Stage 2 (runtime): eclipse-temurin:21-jre with only the JAR

Removes the backend source volume mount and maven_cache named volume.
Deploy with: docker compose up -d --build

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/Dockerfile

@@ -1,9 +1,16 @@
-FROM eclipse-temurin:21-jdk
-
+FROM eclipse-temurin:21-jdk AS builder
 WORKDIR /app
 
-EXPOSE 8080
+# Copy wrapper and POM first — dependency layer is cached separately from source
+COPY .mvn .mvn
+COPY mvnw pom.xml ./
+RUN --mount=type=cache,target=/root/.m2 ./mvnw dependency:go-offline -q
 
-# Source code and mvnw are mounted via docker-compose volume at runtime.
-# Maven dependencies are cached in a named volume (~/.m2).
-CMD ["./mvnw", "spring-boot:run"]
+COPY src ./src
+RUN --mount=type=cache,target=/root/.m2 ./mvnw clean package -DskipTests -q
+
+FROM eclipse-temurin:21-jre
+WORKDIR /app
+COPY --from=builder /app/target/*.jar app.jar
+EXPOSE 8080
+CMD ["java", "-jar", "app.jar"]